I. Field of the Invention
The present invention relates to a single-mode fiber designed for large capacity transmission system. The fiber has low dispersion slope, moderate dispersion, low attenuation, and excellent bend resistance performance. It is suitable for a high speed (10 Gbits/s and 40 Gbits/s), large capacity, long distance dense wavelength division multiplex (DWDM) system, and all the S, C, and L bands of the fiber can be used for DWDM transmission.
II. Description of the Related Art
Since the middle of nineties of the twentieth century, along with the developments of the erbium-doped fiber-amplifier (EDFA) and wavelength division multiplex (WDM) technology, the fiber communication enters its unprecedented high-speed developing stage. The capacity of the fiber communication increases doubly every year. Today, the bit rate of the commercial transmission system reaches 10 Gbits/s, and the capacity reached 1.6 T. When the transmission rate exceeds 2.5 Gbits/s, and because of the practicability of the EDFA, dispersion, instead of attenuation, becomes the main limited factor of long distance transmission. Along with the development of the WDM technology, the influence on the system from the non-linear effects (for example, four-wave mixing, self-phase modulation, cross phase modulation, etc.) among the respective wavelengths limits the augment of the system capacity. In order to restrain the influence of the non-linearity in the DWDM system, a proper amount of dispersion is necessary in its transmission band. Therefore, the development of fiber technology transfers from zero dispersion-shifted fiber to non-zero dispersion-shifted fiber. A number of methods for designing and producing such fiber have been disclosed, for example, a non-zero dispersion-shifted fiber having larger effective area and method for producing the same has disclosed in Chinese Patent 98121639.0, such fiber has been widely used in the construction of communication backbone network. The further growth of transmission capacity impels the consideration of sufficiently making use of bandwidth resource of fiber. The range of utilizing amplifier has extended from C-band to L-band. At the same time the breakthrough in researching new amplifiers that can be used in wider wavelength range has been realized. It has been reported that the product of gain and bandwidth of Raman fiber amplifier (RFA) reaches 132 nm now, and a gain of 30 dB in the range of 1480 nm˜1620 nm can be obtained. The C-band and L-band RFA modules that can be used in 40 Gbits/s system are available commercially. However, the zero dispersion point of the current non-zero dispersion-shifted fiber is within S-band, so that S-band cannot be used for DWDM transmission. In addition, the dispersion slope in C-band and L-band is somewhat great, and its value exceeds 0.08 ps/nm2·km. Thus, when making management of dispersion using dispersion compensation technique, except in central wavelength region, the complete compensation cannot be obtained in side band regions. The wider wavelength region for transmission shall result in higher residual dispersion in the side band region. This problem does not affect greatly a system which transmission rate is less than 10 Gbits/s, however, for a 40 Gbits/s high-speed transmission system that requires precisely managing dispersion, the high dispersion slope becomes a serious problem. Therefore, lowering the dispersion slope of fiber is required, so as to decrease the difference between the dispersions of long wavelength and short wavelength that increases along with the distance, and the bandwidth can be fully utilized. Because the non-linear effect affects a large capacity high-speed transmission system more intensively, properly increasing the dispersion value of fiber is required, so as to restrain the influence from the non-linear effect. In the application documents of 00806764.3 in Chinese Patent Publication, there is an embodiment in which a triangular profile that having a central depression is adopted, its dispersion slope decreases to 0.073 ps/nm2·km, but the zero dispersion point is 1499 nm, and not shifted out of S-band, and its dispersion slope is still on the high side. A fiber is disclosed in U.S. Pat. No. 6,396,987B1, its effective area is greater than 60 μm2, its dispersion slope is less than 0.07 ps/nm2·km, its dispersion at 1550 nm ranges from 7 ps/nm·km to 9 ps/nm·km, its zero dispersion point is within 1400 nm˜1440 nm, its cutoff wavelength exceeds 1600 nm, and its attenuation at 1550 nm is less than or equals to 0.23 dB/km. In said U.S. Patent, the refractive index profiles of the core segments are trapezoidal-index profile and step-index profile having central depression. A step-index profile is also given in the application documents of 00802639.4 in Chinese Patent Publication, its dispersion at 1550 nm ranges from 7 ps/nm·km to 15 ps/nm·km, and its dispersion slope is less than 0.07 ps/nm2·km.
However, because the fiber section has a fewer number of segments, its structure is rather simple, and the parameters for controlling the refractive index profiles of the core segments are relatively fewer, so that to precisely control the parameters of fiber is difficult. Therefore, to equilibrate and control the dispersion, dispersion slope, effective area, and attenuation performances is difficult. This situation becomes more prominent in the mass production.
Definitions and Explanations:
Relative refractive index difference Δ%=[(ni2−nc2)/2ni2]×100 in which ni is the refractive index of the ith layer, and nc is the refractive index of the pure silica glass portion of the cladding, it is the reference refractive index in the invention.
Refractive index profile is defined as the relation of the relative refractive index difference Δ% or relative refractive index of a selected portion versus its radius.
Total dispersion is defined as the algebraic sum of waveguide dispersion and material dispersion of the fiber. In the optical fiber communication field, the total dispersion is chromatic dispersion of fiber. Its unit is ps/nm·km
Dispersion slope represents the dependence of dispersion on wavelength, it is a slope of such a curve that the wavelength is taken as its abscissa, and the dispersion value is taken as its ordinate. Its unit is ps/nm2·km. In a WDM system, if the transmission path has large dispersion slope, then the difference of dispersion value between wavelengths increases, and the transmission performance is deteriorated as a whole.
Effective area is defined as:Aoff=2π(∫E2rdr)2/(∫E4rdr)in which the limit of integration is from 0 to ∞, and E is electric field that relates to propagation.
DWDM is the abbreviation of dense wavelength division multiplex.
The bend resistance performance of fiber refers to as the additional attenuation under specified test condition. The standard test condition includes winding 100 turns on a reel having a diameter of 75 mm and winding one turn on a reel having a diameter of 32 mm. The process is as follows: first testing attenuation of fiber under normal test condition; then winding the fiber on a reel and testing its attenuation according to the standard; the difference of these two tested attenuation values is the bend induced additional attention. Generally, the allowable maximum bend induced attenuation takes the bend induced additional attenuation at 1310 nm as the standard, the unit of the bend induced additional attenuation under each test condition is dB. In the present application the method for testing bend induces additional attenuation is more rigorous, i.e., testing the additional attenuations at 1550 nm and 1625 nm under the conditions of winding 100 turns on Φ 60 mm reel and winding one turn on 32 mm reel, and taking the maximum value as the final measured result.